Relaxation oscillators are often used in integrated circuit (IC) design. They are defined as circuits that alternately charge and discharge a timing capacitor between two internally determined threshold voltages. A periodic output voltage waveform is produced to have a frequency that is inversely proportional to the timing capacitor value and proportional to the charging current.
FIG. 1 shows a well known prior art relaxation oscillator. An operational amplifier (op amp) 10 is operated from a split power supply connected + to V.sub.CC rail 11 and - to V.sub.EE rail 12. The ground potential is substantially midway between +V.sub.CC and -V.sub.EE. Output terminal 13 develops a square wave that swings between close to +V.sub.CC and -V.sub.EE. Resistors 14 and 15 form a voltage divider connected to output terminal 13. The tap is connected to the noninverting input of op-amp 10. This feedback is positive so that the circuit is oscillatory. Resistor 16 and capacitor 17 form an integrator connected between output terminal 13 and the inverting op-amp input. When the output is high resistors 14 and 15 establish a positive threshold potential at the noninverting op-amp input. Capacitor 17 will then charge through resistor 16 until the threshold is exceeded at which time the output will go negative. Now resistors 14 and 15 will establish a negative threshold at the inverting input. Capacitor 17 will discharge and then charge towards the negative potential until the negative threshold is reached. Thus, the capacitor will charge positively and negatively between the threshold levels established sequentially by the resistor divider. The oscillator frequency will be determined by the capacitor value, the values of resistors 14-16 and the total supply voltage. In an alternative construction resistor 16 can be replaced by a constant current source.
FIG. 2 shows a relaxation oscillator circuit that operates from a single supply. A pair of cascaded inverters 19 and 20 are operated from a single power supply connected + to V.sub.CC terminal 21 and - to ground terminal 22. A timing capacitor 23 is connected between the output of inverter 19 and the input of inverter 20 thus creating oscillatory feedback. Timing resistor 24 is coupled between the input and output of inverter 20.
When terminal 25 is high and terminal 26 is low, capacitor 23 will charge through resistor 24 until the threshold of inverter 19 is reached at which time terminal 25 will switch low and terminal 26 will switch high. Then capacitor 23 will charge in the other direction through resistor 24 until the threshold of inverter 20 is reached at which time the circuit will again switch. The outputs at terminals 25 and 26 are complementary square waves having amplitudes that swing between close to ground and +V.sub.CC. The frequency is determined primarily by capacitor 23 and resistor 24 and the threshold of inverter 20. As was the case for the FIG. 1 circuit, timing resistor 24 can be replaced with a constant current source.
In both of the above prior art circuits a square wave output is produced and the symmetry of the circuit results in a close to 50% duty cycle. Other prior art relaxation oscillator circuits operate to control the capacitor charge and discharge currents separately so that other duty cycle values can be achieved, but these circuits are typically subject to fabrication variables and display temperature and fabrication process sensitivity that is undesirable.